The present invention relates to printable media vacuum transport systems. More specifically, the present invention relates to rotational air valves for printable media hold-down vacuum transport systems.
Direct-to-paper ink jet printing systems typically include a printable media hold-down system. As a printable medium passes on a transport surface under an ink jet print head, the hold-down system attempts to prevent contact between the printable medium and the print head. Contact between printable media and the print head may result in fibers from printable media becoming lodged in ink nozzles in the print head. Over time, a substantial number of fibers could become lodged in the nozzles causing the print head to clog. A clogged print head can damage printable media by printing incorrectly, waste ink, and cause significant downtime if the clogged head must be cleaned and/or replaced.
Some high speed printing systems, or systems for printing larger sizes of printable media, may require a large array of print heads. A clogged print head is especially troubling when using a print head array. Cleaning and/or replacing the print heads in a print head array can cause an even greater downtime depending on the size of the print head array.
Several hold-down systems are prevalent in modern direct-to-paper printing systems. One example is a vacuum/plenum system. In this system, a series of small holes are placed in the transport surface, and air is sucked through the holes, away from the print head (or print head array). As the printable medium passes under the print head (or print head array), a vacuum is created under the printable medium, thereby holding the printable medium against the transport surface.
Vacuum hold-down systems have inherent problems, however. Specifically, vacuum hold-down systems have limits to the amount of force that can be applied across printable media to protect and prevent the printable media from coming into contact with the print head (or print head array). Vacuum hold-down systems are particularly susceptible to failure at the leading and trailing edges of the media. At the leading and trailing edges, the downward force caused by the vacuum is less than at other portions of a printable medium due to air leakage around the edges of the printable medium. Also, at the corners of the edges, the bending moment imparted by the vacuum is lowest, which can result in the corners bending away from the transport surface and contacting the print head (or print head array).
One approach to eliminate this problem is to use multiple vacuum chambers each having a separate air removal device to reduce any drop in vacuum pressure due to air escaping around the edges of a printable medium. However, the separate air removal devices are expensive and require a large amount of space, and the number of separate air chambers in current systems is limited to the number of separate air removal systems.